The present invention is directed to a flexible packaging composite suitable for storage in roll form having on opposite sides of the composite (which are in contact with one another in roll form) a coating of a release composition formed from a water-soluble or water-dispersible formulation and a coating of a cold seal adhesive.
Cold seal adhesives (CSA), typically acrylic and rubber latex based, have been widely used to close food packages containing heat-sensitive foods such as chocolate bars and candies where heat sealing of the package is not desirable. Cold seal adhesives are typically coated or printed onto a flexible packaging film to allow sealing of the package with pressure. High package production rates are now achieved with cold seal technology, and this has resulted in high growth and increased popularity for packaging snack-foods.
Oriented polypropylene (OPP) film is widely used in cold seal packaging structures. OPP film manufacturers have developed co-extruded films comprised of a cold seal receptive layer, a core layer, and a cold seal release layer and other related structures. These films offered improvement in cold seal release while maintaining low package manufacturing costs. However, CSA manufacturers have continued their developments due to demand for better package sealing features that are required for new applications. These developments include more aggressive adhesives and adhesives that enabled higher packaging rates and reduced package costs. OPP films have not been able to consistenly meet the need for a packaging film that can be efficiently unwound at the high line-speeds required by package converters.
Package printers and converters have overcome these limitations to some extent by a combination of the following actions: reducing roll unwinding speeds; utilizing less aggressive CSA's; or lowering CSA coat-weight. These actions increase costs or restrict packaging usefulness by limiting the sealing properties.
Additionally, some package converters are over-coating the outer surface which does not contain the cold seal adhesive with a cold seal release lacquer. These release lacquers usually require an additional processing step (U.S. Pat. No. 5,466,734) and may contain volatile organic solvents and ultimately increase costs. Release lacquer formulations may contain silicone compounds or other release agents. Although these compounds may give acceptable cold seal release properties, they are not preferred due to a tendency to perform inconsistently. Another problem is that these release agents, depending on environmental storage conditions, may migrate, transfer, and contaminate the surface of some cold seal adhesives and reduce bond integrity and sealing performance. Also, silicone compounds, if utilized in release lacquers, can wipe-off and contaminate packaging machinery and reduce the coefficient of friction (COF) of packaging films to the extent that film tension control, film tracking, and filled package handling can be negatively affected. Various fluorochemical release coatings have been evaluated; in general, these release materials are very expensive and suffer from poor adhesion to plastic packaging films; this results in transfer and deadening of the CSA.
Other limitations of existing CSA release films are related to poor dimensional and thermal stability. High drying temperatures are required by converters and printers to evaporate the water and remove residual volatile organic compounds (VOC) from the CSA; however, this causes web distortion and registration difficulties with OPP webs due to stretching and shrinkage of the substrate. VOC's must be removed to ensure good seal integrity and reduce chances of adding unwanted odors or contamination to the food.
Roll storage environmental conditions and the pressure created by the roll winding tension can cause blocking, or an interaction between the CSA and an opposite surface of the film. The roll is subsequently unwound at high manufacturing speeds where the following problems may occur: "legging"or stringing of the CSA; transfer of the release agent (1) to the CSA; tearing, stick/release, or "wrap-ups"of the entire laminate; unwanted noise generation as the surfaces of the roll are separated. There is a need for a packaging film that gives improved high speed release of CSA's when rolls are unwound at high speed.